Subterranean drilling systems that employ downhole drilling motors are commonly used for drilling boreholes in the earth for oil and gas exploration and production. A subterranean drilling system typically includes a downhole drilling motor that is operably connected to an output shaft. A pair of thrust-bearing apparatuses also can be operably coupled to the downhole drilling motor. A rotary drill bit configured to engage a subterranean formation and drill a borehole is connected to the output shaft. As the borehole is drilled with the rotary drill bit, pipe sections may be connected to the subterranean drilling system to form a drill string capable of progressively drilling the borehole to a greater depth within the earth.
Each thrust-bearing apparatus includes a stator that may not rotate and a rotor that is attached to the output shaft and that may rotate with the output shaft. The stator and rotor each include a plurality of bearing elements that provide bearing surfaces that bear against each other during use.
In operation, high-pressure drilling fluid is circulated through the drill string and power section of the downhole drilling motor, usually prior to the rotary drill bit engaging the bottom of the borehole, to generate torque and rotate the output shaft and the rotary drill bit attached to the output shaft. When the rotary drill bit engages the bottom of the borehole, a thrust load is generated. This load is commonly referred to as “on-bottom thrust”, which compression load is carried, at least in part, by one of the thrust-bearing apparatuses. Fluid flow through the power section may cause what is commonly referred to as “off-bottom thrust,” which may be carried, at least in part, by the other thrust-bearing apparatus. The drilling fluid used to generate the torque for rotating the rotary drill bit exits openings formed in the rotary drill bit and returns to the surface, carrying cuttings of the subterranean formation through an annular space between the drilled borehole and the subterranean drilling system. The drilling fluid may also serve to cool and lubricate the bearing elements of the thrust-bearing apparatus.
The on-bottom and off-bottom thrust carried by the thrust-bearing apparatuses can be extremely large. The operational lifetime of the thrust-bearing apparatuses often determines the useful life of the subterranean drilling system. Therefore, manufacturers and users of subterranean drilling systems continue to seek improved bearing apparatuses and operation methods to extend the useful life of such bearing apparatuses.